The rotator cuff us a sleeve of four muscles that attach the proximal humerus to the scapula. Its primary role is to initiation of shoulder movement and control of the arm position in space. Rotator cuff tears (RCT) are increasingly common in the aging population, with some studies showing that up to 20% of the population greater than 50 years of age has a symptomatic RCT. Patients will symptomatic RCT have difficulties raising their arms and many of their daily activities are impaired as a result. Our VA patients are an advanced aging population, compared to general population in U.S. With the aging of our Veteran population, RCT is becoming a more and more important health issue for the VA patients. The development of muscle fibrosis and fatty infiltration (FI) are critical factors that determine the clinical outcome of patients with tis injury. It has been demonstrated that the amount of FI in rotator cuff muscles correlate with poor clinical results. There is no effective strategies currently exist to treat this pathologic change once it occurs. Recent works from multiple groups have suggested that a population of muscle progenitor cells, named fibro-adipo progenitor cells (FAPs) are responsible for muscle fibrosis and FI after direct muscle injuries. However, the role of FAPs in rotator cuff muscle pathology has not been studied. The transforming growth factor beta (TGF?) and bone morphogenetic protein (BMP) signaling pathways have been reported to play critical roles in regulating stem cell differentiation, including fibrogenesis and adipogenesis. However, their role in regulating FAPs differentiation in rotator cuff muscle fibrosis and FI remains unknown. Better understanding of the role of these pathways in rotator cuff muscle fibrosis and fatty infiltration may lead to novel pharmacological treatment to prevent or reserve muscle pathology after RCT, thus to improve the clinical outcomes for our effect Veterans.